Injun Joe is Tom Sawyer’s villain. His actions are motivated, from beginning to end, by unadulterated malevolence. When Injun Joe explains his motivation for revenge against Dr. Robinson and later against the Widow Douglas, we see that his personal history involves others mistreating and excluding him. Yet the disproportion between the wrongs Injun Joe has suffered—at least as he enumerates them—and the level of vengeance he hopes to exact is so extreme that we aren’t tempted to excuse his behavior. In contrast, Muff Potter’s misdeeds are inconsequential compared to the punishment he stands to receive. One might also compare Injun Joe to Sid: both are motivated by malice, which they paper over with a convincing performance of innocence.
Though his appearance changes when he disguises himself as a deaf and mute Spaniard, Injun Joe undergoes no real character development over the course of the novel. He never seems to repent for his crimes or change his spiteful outlook. His reappearances in different parts of the novel help to provide a thread of continuity, as they bring the murder-case plot, the treasure-hunt plot, and the adventures-in-the-cave plots together into a single narrative. Injun Joe’s presence also adds suspense to the novel, because we have very little sense of whether Tom and Huck’s constant fear that Injun Joe will hurt them has any basis in reality.